The present invention relates to a lane marker recognition method that uses road surface information obtained through a camera or other input means to recognize through image processing a vehicle running lane marker, and an apparatus using the same.
A technique has conventionally been proposed in which image processing is employed to recognize a white line or other lane marker placed on a road so as to give a driver a warning if his or her vehicle deviates from a cruising lane. Another technique proposed uses a result of lane marker recognition for steering control of the vehicle.
To recognize a white line through image processing, a common conventional approach sets a threshold value for luminance of an image so as to binarize the image, thereby recognizing a portion with a high luminance value as the white line. This approach uses a principle, in which, since the white line has a higher lightness than a surrounding road area, it accordingly exhibits a higher luminance value than the road area on the image. Setting the threshold value in luminance between the road area and the white line area allows the white line area only to be detected through binarization.
If an image has a shadowy white line, however, the shadowed portion of the white line exhibits a lower luminance than the remaining portion of the white line. This hampers proper detection of the shadowed portion of the white line by this approach. There is still another problem, in which, if the threshold value is set at the luminance level that allows the approach to detect the shadowed portion of the white line, this causes the approach to detect also areas other than the white line.
Japanese Patent Laid-open No. Hei 4-152406 discloses a technique, in which the threshold value is set based on a mean value and a maximum value of luminance of an entire image. This technique allows the threshold to be changed according to image conditions, which makes possible even more stabilized white line recognition.
There is, however, a problem inherent in this technique. That is, if image conditions change due to weather or shadow, the technique is no longer able to detect the white line properly.
There is another technique used to recognize the white line, in which an edge extraction is performed to recognize an outline of the white line, instead of binarization based on luminance. The edge extraction extracts a portion at which luminance of the image changes. Specifically, the technique uses the fact that the road area surrounding the white line is dark and there is a change in brightness at an edge of the white line. The technique then recognizes the portion, at which this change in brightness occurs, as the edge of the white line. Since the edge extraction is to detect the change in brightness, this offers an advantage that, even if brightness of a wide area of the image changes due to a change in weather, this is still able to detect the white line as long as there is a change in luminance at the edge of the white line. While being capable of extracting edges of the white line stably, the technique using the edge extraction is, however, likely extracts extra edge components on the road, such as a preceding vehicle and a rut.
A technique that discriminates the edge of the white line from others is therefore important.
Japanese Patent Laid-open No. Hei 11-195127 discloses a technique, in which edge angle information that is obtained when an edge point is obtained is used to determine the area enclosed by edge points that make an angle pair as the white line. This technique is based on the fact that a pair of edges comprising a left edge and a right edge of the white line is extracted from the white line and the difference in angle therebetween is approximately 180 degrees. If there are no edge points making an angle pair, the edge points can be eliminated from the white line.
If the distribution of edge points of the white line to be recognized in lane markers forms a linear line, the position angle information of the edge point can be used to estimate the white line portion as in the technique disclosed in Japanese Patent Laid-open No. Hei 11-195127. This conventional technique is not, however, concerned with a road, in which road studs are installed as the lane marker. Thus, it is unable to recognize road studs.
A technique is available for recognizing the road stud, in which a road stud pattern is registered as a template and the location of the road stud is detected through template matching. However, the size of the road stud on the image is generally small and the image of the road stud is highly susceptible to noise. This is a major obstacle to a stabilized recognition rate.
If an algorithm for white lines differs from that for road studs and, to recognize a lane marker on a road, on which both white lines and road studs are installed, it is necessary to correctly identify the type of the lane marker. This means that correct recognition cannot be made if the type of the lane marker is incorrectly identified even with excellent algorithms provided for both the white lines and road studs. Hence, an overall recognition rate is reduced.